Phosphorus, particularly the phosphorus delivered by zinc dialkyldithiophosphate (ZDDP), has been the predominant antiwear agent in fully formulated lubricants for the past 50 years. Studies have suggested that phosphorus may poison catalytic converters used on gasoline-fueled engines to reduce exhaust emissions of unburned hydrocarbons and oxides of nitrogen [Spearot, J. A., and Caracciolo, F. (1977), “Engine Oil Phosphorus Effects on Catalytic Converter Performance in Federal Durability and High Speed Vehicle Tests,” SAE Technical Paper 770637; Caracciolo, F., and Spearot, J. A. (1979), “Engine Oil Additive Effects on the Deterioration of a Stoichiometric Emissions Control (C-4) System,” SAE Technical Paper 790941; Ueda, F., Sugiyama, S., Arimura, K., Hamaguchi, S., and Akiyama, K. (1994), “Engine Oil Additive Effects on Deactivation of Monolithic Three-Way Catalysts and Oxygen Sensors,” SAE Technical Paper 940746]. As the environmental regulations governing tailpipe emissions have tightened, the allowable concentration of phosphorus in engine oils has been significantly reduced. Further reductions in the phosphorus content of engine oil are likely in the next category, GF-5, to perhaps 0.05 wt. %.
Many partial solutions exist, where either Zn, P, or S have been partially or totally eliminated. In one approach Zhang et al. [Zhang, Z., Yamaguchi, E. S., Kasrai, M., Bancroft, G. M., “Tribofilms Generated From ZDDP and ashless dialkyldithiophosphate (DDP) on Steel Surfaces, Part 1, Growth, Wear, and Morphological Aspects,” Tribology Letters, Vol. 19, 3, pp 211-220 (2005)] studied the growth and morphology of tribofilms, generated from ZDDP and a DDP over a wide range of rubbing times (10 seconds to 10 hours) and concentrations (0.1-5 wt. % ZDDP), using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES) spectroscopy at the O, P, and S K-edges and the P, S, and Fe L-edges. The major components of all films, generated using a Cameron-Plint tester, on 52100 steel are Zn and Fe phosphates and polyphosphates. The average thickness of these phosphate films has been measured using P K-edge XANES and XPS profiling. For ZDDP, a very significant phosphate film (about 100 Å thick) forms after 10 seconds, while film development for DDP is substantially slower. However, for both additives, the average film thickness increases to 600-800 Å after 30 minutes of rubbing, before leveling off or decreasing.
The antiwear properties of pure ZDDP and in combination with DDP at different rubbing times and concentrations were also been examined. It was found that under all conditions, the performance of ZDDP as an antiwear agent is superior to that of DDP. However, DDP has no adverse effect on the performance of ZDDP when the two are mixed, suggesting that DDP can be used with ZDDP, thereby reducing the amount of total ash.
Another approach that reduces ash was developed by Manka in U.S. Pat. No. 5,674,820 relates to a composition, comprising: (A) a compound represented by the formula:

wherein
R1, R2, R3, and R4 are independently hydrocarbyl groups, and X1 and X2 are independently O or S, and n is 0 to 3; and (B) an acylated nitrogen-containing compound have a substituent of at least 10 aliphatic carbon atoms. In one embodiment, the inventive composition further comprises (C) a second phosphorus compound other than (A), said second phosphorus compound being a phosphorus acid, phosphorus acid ester, phosphorus acid salt, or derivative thereof. In one embodiment, the inventive composition further comprises (D) an alkali or alkaline earth metal salt of an organic sulfur acid, carboxylic acid, or phenol. In one embodiment, the inventive composition further comprises (E) a thiocarbamate. These compositions are useful in providing lubricating compositions and functional fluids with enhanced antiwear properties. Specifically, the compositions disclosed are useful as tractor hydraulic fluids, which show enhanced antiwear and antiscore performance.
In U.S. Pat. No. 5,405,545, antiwear and antioxidant properties are claimed for this invention. A lubricant additive having antiwear and antioxidant properties is the reaction product of a thiodicarboxylic acid and an ether amine, preferably 3,3′-thiodipropionic acid and N-isoeicosyloxypropyl-1,3-diaminopropane which is post-reacted with an aliphatic alcohol, preferably oleyl alcohol, an aliphatic amine, preferably a tert-C12 to C14 amine and/or a trialkylphosphite, preferably a tributylphosphite. The post-reaction product contains at least one ester, amide, and/or phosphonate functional group. Data from a Four-Ball test were given in support of the beneficial antiwear performance.
A supplemental wear inhibitor that contains no phosphorus is described in U.S. Publication No. 2003/0148899 A1. This disclosure provides a lubricant oil composition, having enhanced wear-preventive characteristics for a diesel engine operating with large quantities of soot in the oil (soot content: 0.20-4.0 wt. %), and is especially suitable for a pressure-accumulating (common rail) type diesel engine equipped with an exhaust gas recirculation (EGR) system. The claimed lubricant oil composition contains a base oil composed of a mineral and/or synthetic oil incorporated with at least three additives that are a sulfurized oxymolybdenum dithiocarbamate at 0.03 to 0.50 wt. % as Mo; a zinc dialkyldithiophosphate at 0.04 to 0.05 wt. % as P; and at least one metallic salt of alkyl salicylate selected from the group consisting of a Ca salt of alkyl salicylate at 0.004 to 1.0 wt. % as Ca, Mg salt of alkyl salicylate at 0.002 to 0.60 wt. % as Mg, and Zn salt of alkyl salicylate at 0.006 to 1.60 wt. % as Zn, all percentages being based on the whole composition. Bench tests in SRV friction/wear tester were conducted.
The above references largely describe P- or S-containing supplemental wear inhibitors. Unfortunately the tightening of emission requirements requires wear inhibitors with no P, S, and Zn. Trialkylsilanes were disclosed to add thermal stability to lubricants in U.S. Pat. No. 4,572,791 and phenyltrialkylsilanes were disclosed for oxidation improvement in U.S. Pat. No. 5,120,485. Trifunctional hydrolyzable silanes have found some applications in fuels and lubricant compositions; U.S. Pat. No. 4,541,838 discloses additive mixtures of an organic nitrate ignition accelerator and a trialkoxysilane for use in fuel compositions. U.S. Pat. No. 6,887,835 discloses bis-(trialkoxysilyl)alkyl polysulfides as well as other linking groups including polysiloxanes. The bis and polymeric silane compounds showed a reduction in the Falex 4-ball wear scar using the ASTM D 4172 test.
Russian Patent No. SU-245955 (Jun. 11, 1969) discloses lubricant additives which improve the antifriction and anticorrosion characteristics of lubricating oils when used in amounts of 2-35% weight, preferably 5% wt are trialkoxyorganosilanes of the general formula (AlkO)3SiRR′ (where AlkO is an alkoxy group, R is alkyl, aryl or alkenyl group, and R′ is a functional group such as such as NH2, CO2H, COH, OH, or CN).
Great Britain Patent No. 1 441 335 discloses lubricant compositions to improve antifatigue containing about 0.01 to 5% weight of a condensation polymer derived from a trialkoxysilane of the formula R—Si(OR1)3 where R is C1-24 alkyl or C2-24 alkoxyalkyl, and R1 is C1-12 alkyl or C2-12 alkoxyalkyl, where alkoxyalkyl means an ether group represented by —Cn—O—Cm— wherein the sum of n plus m is 2 to 24 in the case of R and 2 to 12 in the case of R1.
Japanese Patent Publication No. 8-337788 (Dec. 24, 1996) discloses additives consisting of silane compounds, e.g., a): R1Si(OR)3, b): (R1)2Si(OR)2, and c): (R1)3SiOR where (R=H, C1-18 alkyl, C2-18 alkenyl, C6-18 aryl; and R1=C6-50 alkenyl optionally containing a N, O, and/or S atom or substituted with hydroxyl, carbonyl, alkoxycarbonyl, alkenoxycarbonyl or aryloxycarbonyl, or a C6-50 aryl. Also claimed are (i) lubricating oil compositions containing for engines comprising 0.05-10 wt. % the additive(s); (ii) compositions containing: (A) the additive(s); (B) a metal cleaner(s) in a base oil; (C) an extreme pressure lubricant(s); and (D) an ash-free dispersant(s). The additives are said to improve cleanliness of the piston of engines and thereby allow a reduction of amount of phosphorus-type extreme pressure agents and ester-type oiliness improvers added and prolong the lifetime of engine oils. The compositions are also said to have high friction reducing effects.